This invention relates in general to sensing devices and in particular to a transducer circuit which includes a balanced bridge.
Sensing devices are used in many places to monitor liquid and gaseous pressures present in machinery. Early devices were mechanical and included a direct coupling between the pressurized media and a pressure display unit, typically a Bourdon tube-gauge. As machines became more complex and pressures increased, the coupling between the media being monitored and the gauge became more difficult. Furthermore, the presence of high pressures in control panels raised safety considerations.
The adoption of sensors, or transducers, that generated an electrical signal proportional to a force solved the coupling and safety problems. The electrical signal so generated is easily transmitted to a remotely located electrically activated gauge.
One type of transducer has a Wheatstone bridge circuit mounted upon a resilent diaphragm. The diaphragm is exposed to the force or pressure being measured. The bridge includes resistive elements which change in value as the diaphragm is deflected by applied pressure. The bridge output is connected to a gauge calibrated to read pressure. Typically, the bridge is balanced for zero output voltage when subjected to zero pressure. As pressure increases, the bridge is unbalanced. The unbalanced bridge produces an output voltage proportional to the applied pressure, which causes the gauge to deflect. Amplification may be included to increase the strength of the bridge signal sufficiently to drive the gauge.
While such a device is simple, it is also susceptible to noise present in the voltage supply and in the pressure being monitored, as occurs in vehicular applications. Vehicle battery voltage varies over a wide range with varying conditions and is accompanied by large alternator and other noise components. Furthermore, both bridge components and amplifiers used in the devices tend to drift, especially in high temperature environments.
Various methods have been developed to desensitize measuring circuits to background noise. Some have involved conversion of the bridge output voltage from an analog to a digital signal. One such method includes feedback of a series of digital signals which are converted to an analog signal. The analog signal is then applied to rebalance the bridge. Concurrent with the feedback, a counter is indexed. When the bridge balance is reestablished, a logic control unit causes the counter to output a value indicative of the applied pressure. These devices have proven to be dependable and accurate; however, the use of converters and the associated digital circuitry results in a complex and expensive measuring device. In mass market applications, such as oil pressure sensors for vehicle engines, a simpler, less expensive, but accurate, device is desired.